Temp Guage Spikes to Max Temerature

A while back I had my water pump (maybe the thermostat, not sure) replaced by a shop who's competency has since been proven severely inadequate. Within 5 minutes of leaving said shop in 100+ degree weather I turned my AC on and shortly thereafter the temperature went from normal to 260* After a few seconds to a minute the needle will drop back to normal. This happened on and off the next couple of days. The temp instantly goes from 210-260 (or whatever max temp is on the gauge) and back a few seconds-minutes later. I replaced the temperature sensor and it continues to do the same thing if the weather is hot enough and I drive on the freeway (not a common occurence usually).

Was the radiator not burped correctly? Do I have a faulty gauge? I watched him burp it with some special funnel, but the AC and/or heater were not run while he was doing so.

Thanks for any help!

EDIT: 2000 4.0L in a 1997 xj.

sounds like either the gauge has a bad connection, or there is still some air in there, maybe stuck in the heater core? i dunno, kinda strange, ive always run the heat wide open when adding coolant to a system that has been drained

If the rad cap wasn't replaced, I'd be suspect of that... Changes in the cooling system can bump the pressure up, and if that cap is shot then it'll cause problems.

Thanks. I suspect there is air in there somewhere and when it gets stuck at the temp sensor it shoots up. I'll flush it and burp it and see if it helps.

2000 should self burp. I'd change the thermostat. I'll assume that you have it filled all the way when it is cold, and that you run it and turn it off after it has been hot and it is still full.

My guess would be a sticking thermostat. Whether new or not, the fact that it is cooling back to normal operating temp after spiking implicates the thermostat not is opening soon enough as it warms up.

David Bricker / SYR

David_Bricker said:

My guess would be a sticking thermostat. Whether new or not, the fact that it is cooling back to normal operating temp after spiking implicates the thermostat not is opening soon enough as it warms up.

David Bricker / SYR

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It is literally instantaneous that it goes up and then also when it goes down. I doubt the fluid is even capable of getting that hot that quick, and it certainly can't cool off that fast

Winterbeater - Only the engine/heads are 2000, if everything else is '97 (radiator etc) would it still self burp?

Yeah, you got the open system. Be very careful about overheating too. You have the notorious 0331 head. They also crack without even overheating. You could pick up an infra-red temp gun to actually check the temperatures on the engine and radiator.

Sounds like a bad sensor, bad sensor wire or bad sensor ground (grounded through the engine block), or a bad temp gauge, or gauge connection.

I agree it can not cool back down that fast! I have seen 30 F spikes for a few seconds when a radiator cap opens, bleeds and closes. Usually a sign of a bad cap!

Ecomike said:

Sounds like a bad sensor, bad sensor wire or bad sensor ground (grounded through the engine block), or a bad temp gauge, or gauge connection.

I agree it can not cool back down that fast! I have seen 30 F spikes for a few seconds when a radiator cap opens, bleeds and closes. Usually a sign of a bad cap!

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I haven't had time to do it myself so I took it to a local shop. They said the block and radiator are running at ~180-190 but the computer is showing ~230 (as does the dash gauge). So they are going to replace the sensor and go from there. Thanks for the help!!

Like was mentioned, put a new cap on it too. It's about the most simple thing you can do for the cooling system, but can make a big difference if the old cap has a weak spring.